A vinyl chloride resin is also referred to as polyvinyl chloride or PVC. A vinyl chloride resin softened at 150° C. to 170° C. is an easily processed thermoplastic resin. A vinyl chloride resin has properties such as superior water resistance, chemical resistance and petroleum resistance, and is hard, thus being used in line plates, pumps, tanks, plated water tanks, treated water tanks, etc. A vinyl chloride including a large amount of plasticizer (40% to 80%) is soft, thus being used in sheets, sides, films, tiles, etc.
When a vinyl chloride resin is prepared through mass polymerization, a heating medium such as water is not used unlike in suspension polymerization or emulsion polymerization, and a vinyl chloride monomer (VCM), an initiator, and, as needed, a reaction additive are supplied and polymerized. In this case, a vinyl chloride resin may be obtained without a dying process after reaction. In addition, in regard to mass polymerization, an apparatus therefor is simple, a reaction rate is fast, a yield is high, highly pure polymer may be obtained, and a polymer may be used as it is. However, there are disadvantages such as difficult temperature control due to strong heat release of a polymeric system, extension of molecular weight distribution of a polymer, and uneasy precipitation. Accordingly, in mass polymerization, a heating medium for heat removal does not exist, and thus, it is very important to maintain stable heat removal.
To a reactor for mass polymerization, only vinyl chloride monomer and initiator are provided before initiation of polymerization reaction, and water as a heating medium as in emulsion polymerization or suspension polymerization is not supplied. Accordingly, when a vinyl chloride resin is prepared using mass polymerization, the amount of liquid vinyl chloride monomer in a reaction system decreases and the amount of solid vinyl chloride resin increases, as polymerization proceeds. Accordingly, a solid vinyl chloride resin and a liquid vinyl chloride monomer have a dispersed shape. In particular, since the specific gravity (1.35 to 1.45) of vinyl chloride resin is greater than the specific gravity (approximately 0.97) of vinyl chloride monomer, a vinyl chloride resin predominantly locates in a lower portion of a reactor. As polymerization proceeds, the amount of vinyl chloride monomer decreases and the amount of vinyl chloride resin increases. Accordingly, all vinyl chloride resins generated in an initial reaction step may maintain a shape wherein the vinyl chloride resins are surrounded with vinyl chloride monomers. However, since, as polymerization proceeds, the amount of vinyl chloride resin increases and the amount of vinyl chloride monomer decreases, a solid vinyl chloride resin cannot maintain a shape wherein the solid vinyl chloride resin is surrounded with a liquid, namely, a liquid vinyl chloride monomer. Accordingly, fluidity of a vinyl chloride resin decreases and an average distance between vinyl chloride resin particles is shortened, whereby microparticles may be generated due to excessive coagulation between particles or friction between particles.
In addition, when a liquid vinyl chloride monomer exists in a sufficient amount, vaporization of a liquid vinyl chloride monomer may be induced by properly removing a gaseous vinyl chloride monomer in a reaction system, whereby reaction heat (namely, polymerization heat) is removed and thus proper reaction temperature (namely, polymerization temperature) may be stably maintained. However, when a liquid vinyl chloride monomer is deficient, it is difficult to remove reaction heat and thus local heating may be encountered, whereby coagulation may be induced due to fusion between vinyl chloride resin particles.
The problems described above are directly related to abnormal products (fine-particle products, oversize-particle products, lumpy products due to coagulation, etc.), and generation of a vinyl chloride resin, which causes formation of a poor sphere.
Until today, interests have been mainly focused on collecting fine particles by installing a separate particle separator after reaction, in order to lower the content of fine particles. In this regard, when a screening method generally used to remove fine particles is used, a screen is easily clogged due to poor static characteristics of bulk-polymerization particles. To compensate for this, U.S. Pat. No. 4,963,634 introduces a collection method using air movement. However, the technology is expensive and use of fine particles collected therefrom is not proper. Accordingly, the method is not a fundamental solution.
Meanwhile, fine particles comprised in a vinyl chloride resin float in the air, and harmfully affect skin and respiratory organs of workers. In addition, upon processing, fine particles melt faster than other particles. When heat is continuously applied, the fine particles are carbonized and thus a surface appearance of a molded product becomes poor or whiteness is decreased. In addition, carbonized particles function as a defect in a molded product and thus, when impact is applied, the carbonized particles function as a start point or an expansion point of cracks. Accordingly, impact strength may be decreased, whereby technology of decreasing such fine particles is very important.